U.S. patent application number 11/651239 was filed with the patent office on 2008-06-05 for systems and methods for rapid uplink air interface synchronization.
This patent application is currently assigned to Adaptix, Inc.. Invention is credited to Xuan Li, Manyuan Shen, Guanbin Xing.
Application Number | 20080130766 11/651239 |
Document ID | / |
Family ID | 39475720 |
Filed Date | 2008-06-05 |
United States Patent
Application |
20080130766 |
Kind Code |
A1 |
Li; Xuan ; et al. |
June 5, 2008 |
Systems and methods for rapid uplink air interface
synchronization
Abstract
Rapid uplink synchronization is enabled by reducing a 2D search
problem to two 1D search problems, which can generally be performed
in less time. Advantage is taken of fact that a mobile device sends
a ranging code on multiple sub-carriers. Using the assumption that
adjacent sub-carriers will have approximately equivalent channel
characteristics, phase ambiguity can be removed by differentially
combining pairs of adjacent sub-carriers. Once the phase ambiguity
is removed, the code, timing, and power level may be determined
relatively quickly. In one embodiment, the values of correlations
between received signals and possible codes are compared with a
threshold.
Inventors: |
Li; Xuan; (Shanghai, CN)
; Shen; Manyuan; (Sunnyvale, CA) ; Xing;
Guanbin; (Issaquah, WA) |
Correspondence
Address: |
FULBRIGHT & JAWORSKI L.L.P
2200 ROSS AVENUE, SUITE 2800
DALLAS
TX
75201-2784
US
|
Assignee: |
Adaptix, Inc.
Seattle
WA
|
Family ID: |
39475720 |
Appl. No.: |
11/651239 |
Filed: |
January 9, 2007 |
Current U.S.
Class: |
375/260 ;
375/343; 375/371 |
Current CPC
Class: |
H04W 52/08 20130101;
H04W 52/42 20130101; H04L 5/023 20130101; H04L 27/261 20130101;
H04W 52/146 20130101; H04L 27/2655 20130101; H04L 27/2662
20130101 |
Class at
Publication: |
375/260 ;
375/371; 375/343 |
International
Class: |
H04L 7/00 20060101
H04L007/00; H04L 27/28 20060101 H04L027/28; H04L 27/22 20060101
H04L027/22 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 30, 2006 |
CN |
200610160842.9 |
Claims
1. A method of establishing an air interface communication between
a base station and a mobile device; said method comprising:
removing phase ambiguity from ranging signals transmitted over a
plurality of sub-carrier signals by said mobile device and received
at said base station; and when said phase ambiguity has been
removed, determining identifying codes sent by said mobile
device.
2. The method of claim 1 further comprising: determining a
propagation time between said mobile device and said base station;
and determining an acceptable power level for air interface
communication between said base station and said mobile device.
3. The method of claim 2 wherein said determining a propagation
time comprises: phase detection.
4. The method of claim 2 wherein said determining a propagation
time comprises a method selected from the list including: inverse
Fast Fourier Transform (FFT) and sine wave correlation.
5. The method of claim 1 wherein said removing phase ambiguity
comprises: differentially multiplying adjacent ranging
sub-carriers.
6. The method of claim 1 wherein said determining identifying codes
comprises: correlating said signals with possible codes; and
comparing correlated values with a threshold.
7. The method of claim 6 wherein said correlating comprises:
multiplying said sub-carriers with local replicas of said possible
codes; and summing results of said multiplication.
8. The method of claim 1 further comprising: instructing said
mobile device to increase transmission power if no identifying code
is determined.
9. The method of claim 1 for use in an orthogonal frequency
division multiple access (OFDMA) system.
10. The method of claim 1 for use in an orthogonal frequency
division multiplexing (OFDM) system.
11. An air interface communication system comprising: means for
removing phase ambiguity from ranging signals transmitted over a
plurality of sub-carrier signals by said mobile device and received
at said base station; and means for, when said phase ambiguity has
been removed, determining identifying codes sent by said mobile
device.
12. The system of claim 11 further comprising: means for
determining a propagation time between said mobile device and said
base station; and means for determining an acceptable power level
for air interface communication between said base station and said
mobile device.
13. The system of claim 12 wherein said means for determining a
propagation time uses phase detection.
14. The system of claim 12 wherein said means for determining a
propagation time uses one of: inverse Fast Fourier Transform (FFT)
and sine wave correlation.
15. The system of claim 11 wherein said means for removing phase
ambiguity comprises: means for differentially multiplying adjacent
ranging sub-carriers.
16. The system of claim 11 wherein said means for determining
identifying codes comprises: means for correlating said signals
with possible codes; and means for comparing correlated values with
a threshold.
17. The system of claim 16 wherein said means for correlating
comprises: means for multiplying said sub-carriers with local
replicas of said possible codes; and means for summing results of
said multiplication.
17. The system of claim 11 further comprising: means for
instructing said mobile device to increase transmission power if no
identifying code is determined.
19. The system of claim 11 wherein said communication system is an
orthogonal frequency division multiple access (OFDMA) system.
20. The system of claim 11 wherein said communication system is an
orthogonal frequency division multiplexing (OFDM) system.
Description
RELATED APPLICATIONS
[0001] This application is related to and claims priority to
Chinese Application No. 200610160842.9 filed Nov. 30, 2006 entitled
"SYSTEMS AND METHODS FOR RAPID UPLINK AIR INTERFACE
SYNCHRONIZATION", the disclosure of which is hereby incorporated
herein by reference.
TECHNICAL FIELD
[0002] This invention relates to air interface communication
systems synchronization between base stations and mobile devices
and more particularly to rapid uplink synchronization based on
signals sent from the mobile devices.
BACKGROUND OF INVENTION
[0003] In wireless (air interface) communication systems, signals
transferred from a plurality of mobile devices arrive at the base
station with different propagation delays and different power.
Large propagation delay and power difference often result in
significant loss of signal at the base station. One method for the
base station to control the propagation delays and power levels of
the signal from mobile devices is to have each mobile device send a
predetermined pseudo random code identifying itself on a defined
ranging time slot or channel. These codes, or ranging signals are
used by the base station (which could include any suitable distant
end transmission point) to determine the time delay and
transmission power level of the mobile device.
[0004] Since the base station does not know which code is being
sent by the mobile device, the base station must isolate the
sub-channel codes for each mobile device. One method of isolating
the code from a mobile device is to match the incoming signal
against a known signal in order to determine which code is being
sent. However, because there are many possible codes and because
they are not arriving at the base station with a known time
(phase), the solution to the problem becomes a two-dimensional
calculation, i.e., first the system must check to see if the signal
contains a known code at a first time (first phase). If not, then
the system must repeat the process for successive time slices
(phases) to see if a particular code is being received. This is
time consuming and requires high processor resources. Besides, the
channel phase ambiguities acting on ranging channel will
significantly deteriorate the measurement precise of propagation
delays.
BRIEF SUMMARY OF THE INVENTION
[0005] A two-dimensional (2D) search problem is reduced to two
one-dimensional (1D) search problems, which can generally be
performed in less time. Advantage is taken of fact that each mobile
device sends the randomly selected ranging code on multiple
sub-channels. In Orthogonal Frequency Division Multiple Access
(OFDMA) and Orthogonal Frequency Division Multiplexing (OFDM)
systems, the ranging channel is often composed of a group of
adjacent sub-carriers. An assumption can then be made that adjacent
sub-carriers (because they are close in frequency and other
characteristics) will have approximately (although not necessarily)
same channel characteristics. By differentially multiplying pairs
of adjacent received ranging sub-carriers, the channel phase
ambiguity can be removed between those sub-carriers. Power levels
for each ranging code can be calculated by correlating the
differential received ranging sub-carriers with local predetermined
differential ranging codes. All the ranging codes with power
meeting a predetermined threshold are selected as the ranging codes
transmitted from the mobile devices. Time delay measurement is then
performed only for the selected ranging codes. Since in most cases
the selected ranging codes belong to a subset of the total ranging
codes, the computing complexity may be reduced.
[0006] The foregoing has outlined rather broadly the features and
technical advantages of the present invention in order that the
detailed description of the invention that follows may be better
understood. Additional features and advantages of the invention
will be described hereinafter which form the subject of the claims
of the invention. It should be appreciated by those skilled in the
art that the conception and specific embodiment disclosed may be
readily utilized as a basis for modifying or designing other
structures for carrying out the same purposes of the present
invention. It should also be realized by those skilled in the art
that such equivalent constructions do not depart from the spirit
and scope of the invention as set forth in the appended claims. The
novel features which are believed to be characteristic of the
invention, both as to its organization and method of operation,
together with further objects and advantages will be better
understood from the following description when considered in
connection with the accompanying figures. It is to be expressly
understood, however, that each of the figures is provided for the
purpose of illustration and description only and is not intended as
a definition of the limits of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] For a more complete understanding of the present invention,
reference is now made to the following descriptions taken in
conjunction with the accompanying drawing, in which:
[0008] FIG. 1 illustrates one embodiment of a flow chart for
obtaining uplink synchronization for air interface communication
between a base station and a mobile device; and
[0009] FIG. 2 shows a typical air interface system in which the
concepts of the invention can be practiced.
DETAILED DESCRIPTION OF THE INVENTION
[0010] FIG. 1 illustrates one embodiment of flow chart 10 for
obtaining uplink synchronization for air interface communication
(23 FIG. 2) between a base station, such as transmission point 201,
shown in FIG. 2, and a mobile device, such as device 21-1 shown in
FIG. 2. The algorithm shown in FIG. 1 can be run, for example, by
processor 241 in conjunction with memory 242 operating in base
station 24 which can be co-located with the actual point of
transmission, or can be remote there from.
[0011] Process 101 extracts the ranging sub-carriers from the
signal received from the mobile device from time to time. In
effect, the ranging sub-carriers are separated from the other data,
such as the payload data, etc. Each ranging channel is composed of
multiple sub-carriers. For example, in an OFDMA system 144 ranging
sub-carriers may be specified. The concepts discussed herein are
particularly well-suited for OFDMA as well as OFDM systems. A
mobile device selects a pseudo-random code and transmits that code
on all of the ranging sub-carriers. The pseudo-random code
identifies the mobile device, and the base station then determines,
as will be discussed below, the average power level of each
received random code in order to determine whether the mobile
device which transmitted that code should adjust its transmission
power.
[0012] Process 102 then differentially multiplies the adjacent
ranging sub-carriers. An approximation may be made that channels of
adjacent sub-carriers are coherent, in the sense that phase
characteristics will be approximately same between adjacent
sub-carriers. This approximation is useful if sub-carrier spacing
is smaller than the channel coherent bandwidth. Channel phase
rotation on each sub-carrier may then be removed by multiplying a
specific sub-carrier with the conjugation of an adjacent
sub-carrier.
[0013] Process 103 multiplies differential ranging sub-carriers
with local replicas of the possible differential ranging codes,
which may be pre-calculated and stored in memory 242 shown in FIG.
2, and then sums the multiplication results. This produces a
correlation between the possible ranging codes and the ranging code
transmitted by the mobile device. The correlation value of the
ranging code transmitted by the mobile device will be highest
value. In this manner, the correlation values can be used by base
station 24, as will be discussed below, to isolate ranging codes
for each mobile device.
[0014] Process 104 calculates the power of the correlation values
for the convenience of threshold comparison in a following process.
Process 105 determines which ranging codes are transmitted by
mobile devices. All of the ranging codes with power exceeding the
predetermined threshold will be selected as the transmitted ranging
codes. If no acceptable power level is found, the mobile device can
be told to increase its power and transmit another ranging code in
subsequence time frame.
[0015] At this point, the code has been identified for certain
mobile device. Process 106 compares the power levels of the
selected ranging codes with target power levels and thus determines
the power adjustment value for that mobile device in subsequent
transmissions. Process 107 calculates time delay using several
methods, such as, for example, phase detection, inverse FFT or sine
wave correlation. This time delay corresponds to the round trip
delay between base station and mobile device, and the mobile device
can use this value to adjust its transmission time in subsequent
frames.
[0016] Transmitted ranging codes may be shown as:
X.sub.1(k,l).epsilon.{-1,1}
where k is ranging sub-carrier index (k=1, . . . ,K) and l is
ranging sequence index (l=1, . . . ,L).
[0017] Received ranging codes in frequency domain is:
X r ( k , l ) = X t ( k , l ) H ( k , l ) - j 2 .pi. k .tau. ( l )
N ##EQU00001##
where H(k,l) is the complex channel transfer function of the k-th
sub-carrier of the l-th transmitted ranging sequence, .tau.(l) is
propagation delay corresponding to the l-th ranging sequence and N
is the sub-carrier number.
R ( l , l ' ) = k [ X r ( k + 1 , l ) X r * ( k , l ) X t ( k + 1 ,
l ' ) X t * ( k , l ' ) ] ##EQU00002## l ' = 1 , 2 , , L
##EQU00002.2##
[0018] Differentially multiplying adjacent ranging codes gives:
X r ( k + 1 , l ) X r * ( k , l ) = X t ( k + 1 , l ) X t * ( k , l
) H ( k + 1 , l ) H * ( k , l ) - j 2 .pi. k .tau. ( l ) N
##EQU00003##
Assuming adjacent channels are coherent, we have:
X r ( k + 1 , l ) X r * ( k , l ) = X t ( k + 1 , l ) X t * ( k , l
) H ( k , l ) 2 - j 2 .pi. k .tau. ( l ) N ##EQU00004##
Multiplying received differential ranging codes with local
differential ranging codes and summing the results yields: Power is
then:
P(l,l')=|R(l,l')|.sup.2
Maximum P(l,l') can be obtained when l'=l, i.e.:
[0019] P max ( l , l ' ) = R ( l , l ' ) l = l ' 2 = R ( l ) 2 = -
j 2 .pi. .tau. ( l ) N k H ( k , l ) 2 = k H ( k , l ) 2 2
##EQU00005##
[0020] Therefore, selection of ranging codes sequences with power P
exceeding the predetermined threshold P.sub.th may be shown as:
L.sub.s={l': P(l,l')>P.sub.th}
One way to calculate the transmit time is the Inverse Fast Fourier
Transform (IFFT) method. The channel impulse response of a ranging
channel is calculated by:
h ( l , t - .tau. ) = IFFT k ( X r ( k , l ) X t ( k , l ) ) = IFFT
k ( H ( k , l ) - j 2 .pi. k r ( l ) N ) ##EQU00006## l .di-elect
cons. L s ##EQU00006.2##
Transmit time delay .tau. may be obtained based on the first path
of the channel impulse response. However, this method requires an
IFFT operation, which may be time consuming and resource intensive.
Another way to estimate the transmit time delay is the phase
detection method, which calculates phase rotation .theta. of a
differential correlation value and obtains transmit time delay
.tau. from:
.theta. ( l ) = arg ( R ( l ) ) = - 2 .pi..tau. ( l ) N
##EQU00007## l .di-elect cons. L s ##EQU00007.2## .tau. ( l ) = -
.theta. ( l ) N 2 .pi. ##EQU00007.3## l .di-elect cons. L s
##EQU00007.4##
[0021] Although the present invention and its advantages have been
described in detail, it should be understood that various changes,
substitutions and alterations can be made herein without departing
from the spirit and scope of the invention as defined by the
appended claims. Moreover, the scope of the present application is
not intended to be limited to the particular embodiments of the
process, machine, manufacture, composition of matter, means,
methods and steps described in the specification. As one of
ordinary skill in the art will readily appreciate from the
disclosure of the present invention, processes, machines,
manufacture, compositions of matter, means, methods, or steps,
presently existing or later to be developed that perform
substantially the same function or achieve substantially the same
result as the corresponding embodiments described herein may be
utilized according to the present invention. Accordingly, the
appended claims are intended to include within their scope such
processes, machines, manufacture, compositions of matter, means,
methods, or steps.
* * * * *